Automatic White Balance
October 25, 2010
It's very tempting to set the white balance control on your DSLR or prosumer video camera to automatic. And there are some excellent excuses for doing so; you keep shooting when the lighting changes, aren’t likely to mistakenly balance on a white card aimed at a secondary light source, and forgetting to balance won't leave you stuck with blue faces or orange-tinted indoor scenes.
Our previous discussions of white balance have all been centered on a simple concept; video cameras need to be told what color light they’re working with. Sunlight has more blue than tungsten. A cloudy day has more blue than sunlight. Shooting in the shade often introduces a greenish component to the light, and so on.
The human brain contains its own white-balancing circuit, automatically giving a consistent “look” to whatever the eye sees. The camera doesn’t. This shortcoming is overcome by pointing the camera at a white wall, car, or piece of paper and pressing a button which tells the camera “Memorize this. This is white.” And everything is fine until the light changes and you have to do the white balance drill again or risk shooting an off-color scene.
Activating an automatic white circuit is somewhat akin to walking around with your finger constantly on the white balance button. The only difference is that when you're shooting a scene you're filling the frame with action, not a white card. And here’s the rub; the auto-white circuit is continuously making the assumption that the brightest portion of the scene is white.
And while that’s often the case, sometimes it's not. So the camera makes a pink shirt white and everything else in the scene shifts blue. Or maybe it's a yellow car passing in and out of frame that causes a transitory color shift. Or a backlit beige window shade that’s far and away the brightest part of the frame. Get the picture?
When it comes to color accuracy, it all boils down to a few simple questions. Do you care that different shots in a sequence might have slightly different color casts, or that the principal subject’s skin tone might shift when a light-colored car drives through the shot? Does the uncontrolled action you are capturing move from indoors to sunlight in the same shot? Will you have time to “fix it in post”? In other words; Is close enough good enough?